Turbine Engine Fundamentals: Brayton Cycle, Thrust, Efficiency and Station Numbering
What this domain tests
The exam checks that you can apply the ideal Brayton cycle to a real turbofan or turboprop, explain where thrust comes from, and read the standard engine station numbers used on diagrams and in fault reports.
Core rules
- The Brayton cycle has four processes: ram compression (intake), mechanical compression, constant-pressure heat addition (combustor), and expansion (turbine and nozzle). The turbine extracts work to drive the compressor; the remaining energy appears as jet velocity and pressure thrust.
- Ideal thermal efficiency depends only on pressure ratio: η = 1 − 1/PR^((γ−1)/γ). Higher pressure ratio and higher turbine inlet temperature raise efficiency, but both are limited by materials and cooling.
- Thrust = mass flow × (Vjet − Vflight) + Ae(Pe − Pamb). At high flight speed the velocity difference shrinks, so propulsive efficiency rises and specific fuel consumption falls.
- Station numbers: 1 inlet, 2 compressor or fan inlet, 25 LP compressor exit, 3 HP compressor exit, 4 combustor exit (turbine inlet), 45 turbine exit, 5 LP turbine exit, 7 exhaust nozzle, 9 nozzle exit. Bypass ducts use 12 to 16.
Worked example
An engine has an overall pressure ratio of 30 and γ = 1.4. The exponent is (1.4 − 1)/1.4 = 0.2857. Then 30^0.2857 ≈ 2.64, so η = 1 − 1/2.64 ≈ 0.62, or 62%. Reasoning: only pressure ratio appears in the ideal formula; real efficiency is lower because of compressor and turbine losses, and because heat addition is not perfectly constant-pressure.
Traps
- Confusing thermal efficiency (fuel energy to shaft or jet power) with propulsive efficiency (jet power to thrust power).
- Mixing N1 and N2 when describing station numbers or spools.
- Assuming maximum thrust always occurs at the highest possible pressure ratio; bleed air, temperature and surge margin intervene.
Key points
- Brayton cycle: compression, constant-pressure heat addition, expansion; the turbine drives the compressor.
- Ideal thermal efficiency rises with pressure ratio: η = 1 − PR^−(γ−1)/γ.
- Thrust = ṁ(Vj − V0) + Ae(Pe − Pamb).
- Station numbers: 2 fan inlet, 3 HPC exit, 4 turbine inlet, 5 turbine exit, 7 nozzle.
- Propulsive efficiency improves at high flight speed; specific fuel consumption falls.